Thermal Printing Technologies
Printing technologies that employ the controlled application of thermal energy via a contacting printhead to activate either physical or chemical image formation processes come under this general classification.
There are four thermal technologies in current use: direct thermal, direct thermal transfer, dye diffusion thermal transfer, and resistive ribbon thermal transfer.
Direct Thermal
This is the oldest and most prolifically applied thermal technology. The imaging process relies on the application of heat to a thermochromic layer of approximately 10 µm in thickness coated onto a paper substrate. The thermally active layer contains a leuco dye dispersed along with an acid substance in a binder. Upon heating fusion melting occurs resulting in a chemical reaction and conversion of the leuco dye into a visible deeply colored mark. Key to this process is the design of the printhead, which can be either a page-wide array or a vertical array or a scanning printhead. Two technologies are in use, thick film and thin film. Thick-film printheads have resistor material between 10 and 70 µm. The resistive material is covered with a glass layer approximately 10 µm thick for wear resistance. The thin-film printheads bear a strong resemblance to those found in thermal ink jet printheads. They employ resistive material, such as tantalum nitride, at 1 µm thickness and a 7-µm-thick silicon dioxide wear layer. Thin-film heads are manufactured in resolutions up to 400 dpi. In each case the resistors are cycled via electrical heating pulses through temperature ranges from ambient (25◦C) up to 400◦C. Overall, the thin-film printheads excel in energy efficiency conversion, print quality, response time, and resolution. For these reasons the thin-film printheads are used when high resolution is required, whereas the thick-film printhead excels in commercial applications such as bar coding, airline tickets, fax, etc.
Direct Thermal Transfer
These printers transfer melted wax directly to the paper (Fig. 19.76(a) and Fig. 19.76(b)). The wax that contains the colorant is typically coated at 4 µm thickness onto a polyester film, which, in common implementations, is approximately 6 µm thick. A thermal printhead of the kind described previously presses this ribbon, wax side down, onto the paper. As the individual heating elements are pulsed, the wax melts and transfers by adhesion to the paper. The process is binary in nature; but, by the use of shaped resistors, which produce current crowding via an hourglass shape, for example, the area of wax transferred can be modulated. Common implementations employ page width arrays at 300 dpi with some providing vertical addressability of 600 dpi. The thermal ribbons are also packaged in cassettes for scanning printhead designs in desktop and portable printers. Power consumption is an issue for all thermal printers, and efforts to reduce this for direct thermal transfer have focused on reducing the thickness of the ribbon.
Dye Diffusion Thermal Transfer
This technology involves the transfer of dye from a coated donor ribbon to a receiver sheet via sublimation and diffusion, separately or in combination. The amount of dye transferred is proportional to the
amount of heat energy supplied; therefore, this is a continuous tone technology. It has found application as an alternative to silver halide photography, graphics, and prepress proofing. As with all thermal printers the energy is transferred via a transient heating process. This is governed by a diffusion equation and depending on the length of the heating pulse will produce either large temperature gradients over very short distances or lesser gradients extending well outside the perimeter of the resistor. Much of the design, therefore, focuses on the thicknesses of the various layers through which the heat is to be conducted. In the case of thermal dye sublimation transfer a soft-edged dot results, which is suitable for images but not for text. Shorter heating pulses will lead to sharper dots.
Resistive Ribbon Thermal Transfer
This technology is similar to direct thermal transfer in that a thermoplastic ink is imaged via thermal energy onto the substrate. The ribbon is composed of three layers: An electrically conductive substrate of polycarbonate and carbon black (16 µm thick), an aluminum layer 1000–2000 A˚ , and an ink layer which is typically 5 µm. The aluminum layer serves as a ground return plane. Heat is generated by passing current from an electrode in the printhead in contact with the ribbon substrate through the polycarbonate/carbon layer to the aluminum layer. The high pressure applied through the printhead ensures intimate contact with the paper, which does not have to be especially smooth for successful transfer. Printed characters can be removed by turning on all electrodes at a reduced energy level and heating the ink to the point that it bonds to the character to be corrected but not to the paper. The unwanted character is removed as the printhead passes over it. This technology does not adapt to color printing in a straightforward way.
Electrophotographic Printing Technology
Electrophotography is a well established and versatile printing technology. Its first application was in 1960 when it was embodied in an office copier. The process itself bears a strong resemblance to offset lithography. The role of the printing plate is played by a cylindrical drum or belt coated with a photoconductor (PC) on which is formed a printing image consisting of charged and uncharged areas. Depending on the implementation of the technology either the charged or uncharged areas will be inked with a charged, pigmented powder known as toner. The image is offset to the paper either by direct contact or indirectly via a silicone-based transfer drum or belt (similar to the blanket cylinder in offset lithography). Early copiers imaged the material to be copied onto the photoconductor by means of geometrical optics. Replacing this optical system with a scanning laser beam, or a linear array of LEDs, which could be electronically modulated, formed the basis of today’s laser printers. As a technology it spans the range from desktop office printers (4–10 ppm) to high-speed commercial printers (exceeding 100 ppm). Although capable of E-size printing its broadest application has been in the range of 8 1 in to 17 in wide, in color and in black and white.
Printing Process Steps
Electrophotographic printing involves a sequence of interacting processes which must be optimized col- lectively if quality printing is to be achieved. With respect to Fig. 19.77 they are as follows.
1. Charging of the photoconductor to achieve a uniform electrostatic surface charge can be done by means of a corona in the form of a thin, partially shielded wire maintained at several kilovolts with respect to ground (corotron). For positive voltages, a positive surface charge results from ionization in the vicinity of the wire. For negative voltages, negative surface charge is produced but by a more complex process involving secondary emission, ion impact, etc., that makes for a less uniform discharge. The precise design of the grounded shield for the corona can have a significant effect on the charge uniformity produced. To limit ozone production, many office printers (<20 ppm) employ a charge roller in contact with the photoconductor. A localized, smaller discharge occurs in the gap between the roller and photoconductor, reducing ozone production between two and three orders of magnitude.
2. The charged photoconductor is exposed as described previously to form an image that will be at a significant voltage difference with respect to the background. The particular properties of the
photoconductor in this step relate to electron hole generation by means of the light and the transport of either electron or hole to the surface to form the image. This process is photographic in nature and has a transfer curve reminiscent of the H and D curves for silver halide. The discharge must be swift and as complete as possible to produce a significant difference in voltage between charged and uncharged areas if optimum print quality is to be achieved. Dark decay must be held to a minimum and the PC must be able to sustain repeated voltage cycling without fatigue. In addition to having adequate sensitivity to the dominant wavelength to the exposing light, the PC must also have a wear-resistant surface, be insensitive to fluctuations in temperature and humidity, and release the toner completely to the paper at transfer. It is possible for either the discharged or the charged region to serve as the image to be printed. Widespread practice today, particularly in laser printers, makes use of the discharged area.
Early PCs were sensitive to visible wavelengths and relied on sulfur, selenium, and tellurium alloys. With the use of diode laser scanners, the need for sensitivity in the near infrared has given rise to organic photoconductors (OPC), which in their implementation consist of multiple layers, including a submicron- thick charge generation layer and a charge transport layer in the range of 30 µm thick. This enables the optimization of both processes and is in wide-spread use today. A passivation or wear layer is used for OPCs, which are too soft to resist abrasion at the transfer stage. In many desktop devices the photoconductive drum is embodied in a replaceable cartridge containing enough toner for the life of the photoconductor. This provides a level of user servicing similar to that for thermal ink jet printers having replaceable printheads.
3. Image formation is achieved by bringing the exposed photoconductor surface in contact with toner particles, which are themselves charged. Electrostatic attraction attaches these particles to form the image. Once again, uniformity is vital, as well as a ready supply of toner particles to keep pace with the development process. Two methods are in widespread use today: dual component, popular for high-speed printing and monocomponent toners commonly found in desktop printers. Dual component methods employ magnetic toner particles in the 10-µm range and magnetizable carrier beads whose characteristic dimension is around 100 µm. Mechanical agitation of the mixture triboelectrically charges the toner particles, and the combination is made to form threadlike chains by means of imbedded magnets in the development roller. This dense array of threads extending from the development roller is called a magnet brush and is rotated in contact with the charged photoconductor (Fig. 19.75). The toner is then attracted to regions of opposite charge and a sensor-controlled replenishment system is used to maintain the appropriate ratio of toner to carrier beads.
Monocomponent development simplifies this process by not requiring carrier beads, replenishment system, and attendant sensors. A much more compact development system results, and there are two implementations: magnetic and nonmagnetic. Magnetic methods still form a magnetic brush but it consists of toner particles only. A technique of widespread application is to apply an oscillating voltage to a metal sleeve on the development roller. The toner brush is not held in contact with the photoconductor but, rather, a cloud of toner particles is induced by the oscillating voltage as particles detach and reattach depending on the direction of the electric field. Nonmagnetic monocomponent development is equally popular in currently available printers. There are challenges in supplying these toners in charged condition and at rates sufficient to provide uniform development at the required print speed. Their desirability derives from lower cost and inherent greater transparency (for color printing applications) due to the absence of magnetic additives.
One way of circumventing the limitations on particle size and the need for some form of brush technique is to use liquid development. The toner is dispersed in a hydrocarbon-based carrier and is charged by means of the electrical double layer that is produced when the toner is taken into solution. Typically, the liquid toner is brought into contact with the photoconductor via a roller. Upon contact, particle transport mechanisms, such as electrophoresis, supply toner to the image regions. Fluid carryout is a major challenge for these printers. To date this has meant commercial use where complex fluid containment systems can be employed. The technique is capable of competing with offset lithography and has also been used for color proofing.
4. The transfer and fuse stage imposes yet more demands on the toner and photoconductor. The toner must be released to the paper cleanly and then fixed to make a durable image (fusing). The majority of fusing techniques employ heat and pressure, although some commercial systems make use of radiant fusing by means of zenon flash tubes. The toner particles must be melted sufficiently to blend together and form a thin film, which will adhere firmly to the substrate. The viscosity of the melted toner, its surface tension, and particle size influence this process. The design challenge for this process step is to avoid excessive use of heat and to limit the pressure so as to avoid smoothing, that is, calendering and/or curling of the paper. Hot-roll fusing passes the toned paper through a nip formed by a heated elastomer-coated roller in contact with an unheated opposing roller that may or may not have an elastomer composition. Some designs also apply a thin film of silicone oil to the heated roller to aid in release of the melted toner from its surface. There is inevitably some fluid carryout under these conditions, as well as a tendency for the silicone oil to permeate the elastomer and degrade its physical properties. Once again materials innovation plays a major role in electrophotography.
5. The final phase involves removal of any remaining toner from the photoconductor prior to charging and imaging for the next impression. Common techniques involve fiber brushes, magnetic brushes, and scraper blades. Coronas to neutralize any residual charge on the PC or background toner are also typical components of the cleaning process. The toner removed in this step is placed in a waste toner hopper to be discarded. The surface hardness of the PC plays a key role in the efficiency of this step. Successful cleaning is especially important for color laser printers since color contrast can make background scatter particularly visible, for example, magenta toner background in a uniform yellow area.
Dot Microstructure
With respect to image microstructure, the design of the toner material, the development technique, and the properties of the photoconductor play key roles. It is desirable to have toner particles as small as possible and in a tightly grouped distribution about their nominal diameter. Composition of toner is the subject of a vast array of publications and patents. Fundamental goals for toner are a high and consistent charge-to-mass ratio, transparency in the case of color, a tightly grouped distribution, and a minimum, preferably no, wrong-sign particles. The latter are primarily responsible for the undesirable background scatter that degrades the print. Recent developments in toner manufacture seek to control this by means of charge control additives which aid in obtaining the appropriate magnitude of charging and its sign. Grayscale in laser printers is achieved by modulating the pulse width of the diode laser. The shape and steepness of the transfer curve, which relates exposure to developed density, is a function of photoconductor properties, development process, and toner properties. It is possible to produce transfer curves of low or high gradient. For text, a steep gradient curve is desirable, but for images a flatter gradient curve provides more control. Since the stability of the development process is subject to ambient temperature and humidity, the production of a stable grayscale color laser printer without print artifacts is most challenging.
Magnetographic and Ionographic Technologies
These two technologies are toner based but utilize different addressing and writing media. The photoconductor is replaced by a thin magnetizable medium or hard dielectric layer, such as anodized aluminum, which is used in ionographic printers. Magnetographic printers employ a printhead that produces mag- netic flux transitions in the magnetizable media by changing the field direction in the gap between the poles of the printhead. These magnetic transitions are sources of strong field gradient and field strength. Development is accomplished by means of magnetic toner applied via a magnetic brush. The toner parti- cles are magnetized and attracted to the medium by virtue of the strong field gradient. Transfer and fusing proceed in a similar manner to that of electrophotography. Ionographic printers write onto a dielectric coated drum by means of a printhead containing individual electron sources. The electrons are generated in a miniature cavity by means of air breakdown under the influence of an RF field. The electron beam is focused by a screen electrode, and the cavity functions in a manner similar to that of vacuum tube valves. The role of the plate is played by the dielectric coated metal drum held at ground potential. The charge image is typically developed by monocomponent toner followed by a transfix, that is, transfer and fuse operation, often without the influence of heat. Both systems require a cleaning process: mechanical scraping for ionography and magnetic scavenging for magnetography.
System Issues
Processing and communicating data to control today’s printers raises significant system issues in view of the material to be printed. Hardcopy output may contain typography, computer-generated graphics, and natural or synthetic images in both color and black and white. The complexity of this information can require a large amount of processing, either in the host computer or in the printer itself. Applications software programs can communicate with the printer in two ways: via a page description language (PDL), or through a printer command set. The choice is driven by the scope of the printed material. If full page layout with text, graphics, and images is the goal, then PDL communication will be needed. For computer generated graphics a graphical language interface will often suffice. However, many graphics programs also provide PDL output capability. Many options exist and a careful analysis of the intended printed material is necessary to determine if a PDL interface is required.
When processing is done in the host computer, it is the function of the printer driver to convert the outline fonts, graphical objects, and images into a stream of bits to be sent to the printer. Functions that the driver may have to perform include digital halftoning, rescaling, color data transformations, and color appearance adjustments among other image processing operations, all designed to enable the printer to deliver its best print quality. Data compression in the host and decompression in the printer may be used to prevent the print speed being limited by the data rate. Printers that do their own internal data processing contain a hardware formatter board whose properties are often quoted as part of the overall specification for the printer. This is typical for printers with a PDL-based interface. Some of the advantages for this approach include speed of communication with the printer and relieving of the host computer of the processing burden, which can be significant for complex documents.
The increase in complexity of printed documents has emphasized several practical system aspects that relate to user needs: visibility and control of the printed process, font management, quick return to the software application, and printer configuration. The degree of visibility and control in the printing process depends on the choice of application and/or operating environment. Fonts, either outline or bit map, may reside on disk, on computer, or printer read-only memory (ROM). To increase speed, outline fonts in use are rasterized and stored in formatter random-access memory (RAM) or computer RAM. Worst cases exist when outline fonts are retrieved at printing and rasterization occurs on a demand basis. This can result in unacceptably slow printing. If quickness of return to the application is important, printers containing their own formatter are an obvious choice. It is necessary, therefore, to take a system view and evaluate the entire configuration (computer hardware; operating system; application program; interconnect; printer formatter, and its CPU, memory, and font storage) to determine if the user needs will be met.
The need to print color images and complex color shaded graphics has brought issues such as color matching, color appearance, and color print quality to the fore. Color printer configuration now includes choices as to halftoning algorithm, color matching method, and, in some cases, smart processing. The latter refers to customized color processing based on whether the object is a text character, image, or graphic. A further complication arises when input devices and software applications also provide some of these services, and it is possible to have color objects suffer redundant processing before being printed. This can severely degrade the print quality and emphasizes the importance of examining the entire image processing chain and turning off the redundant color processing. Color printer configuration choices focus on a tradeoff between print speed and print quality. Halftoning algorithms that minimize visible texture and high print quality modes that require overprinting consume more processing time. For color images and graphics, the relationship between the CRT image and hard copy is a matter of choice and taste. For color graphics, it is common practice to sacrifice accuracy of the hue in the interests of colorfulness or saturation of the print. In the case of natural images, hue accuracy, particularly for flesh tones, is more important, and a different tradeoff is made. Some software and hardware vendors provide a default configuration that seeks to make the best processing choice based on a knowledge of the content to be printed. If more precise control is desired, some understanding of the color reproduction issues represented by the combination of color input and output devices linked by a PC having a color monitor is required. This is the domain of color management.
Color Management
The fundamental issue to be addressed by color management is that of enabling the three broad classes of color devices (input, display, output) to communicate with each other in a system configuration. The technical issue in this is one of data representation. Each device has an internal representation of color information that is directly related to the nature in which it either represents or records that information. For printers it is typically amounts of cyan, magenta, yellow, and often black (CMY, K) ink; for displays, digital counts of red, green, and blue (RGB); and for many input devices, digitized values of RGB. These internal spaces are called device spaces and map out the volume in three-dimensional color space that can be accessed by the device. To communicate between the devices these internal spaces are converted either by analytical models or three-dimensional lookup tables (LUTs) into a device-independent space. Current practice is to use Commision Internationale d’Eclairage (CIE) colorimetric spaces, based on the CIE 1931 standard observer for this purpose. This enables the device space to be related to coordinates that are derived from measurements on human color perception. These conversions are known as device profiles, and the common device independent color space is referred to as the profile connection space (PCS). When this is done it is found that each device accesses a different volume in human color space. For example, a CRT cannot display yellows at the level of saturation available on most color printers. This problem, in addition to issues relating to viewing conditions and the user state of adaptation, makes it necessary to perform a significant amount of color processing if satisfactory results are to be obtained. Solutions to this problem are known as color management methods (CMM) (Fig. 19.78). It is the goal of color management systems to coordinate and perform these operations.
The purpose of a color management system is, therefore, to provide the best possible color preference matching, color editing, and color file transfer capabilities with minimal performance and ease of use penalties. Three levels of color-management solutions are common available, point solutions, application solutions, and operating system solutions. Point solutions perform all processing operations in the device driver and fit transparently into the system. If color matching to the CRT is desired, either information as to the make of CRT or visual calibration tools are provided to calibrate the CRT to the driver. Ap- plication solutions contain libraries of device profiles and associated CMMs. This approach is intended
to be transparent to the peripheral and application vendor. Operating system solutions embed the same functionality within the operating system. These systems provide a default color matching method but also allow vendor-specific CMMs to be used.
Although the creation of a device profile involves straightforward measurement processes, there is much to be done if successful color rendition is to be achieved. It is the property of CIE colorimetry that two colors will match when evaluated under the same viewing conditions. It is rarely the case that viewing conditions are identical and it is necessary to perform a number of adaptations commonly called color appearance transformations to allow for this. A simple example is to note that the illuminant in a color scanner will have a different color chromaticity than the white point of the CRT, which will also differ from the white point of the ambient viewing illuminant. In addition, as has been mentioned, different devices access different regions of color space; that is, they have different color gamuts. Colors outside the gamut of a destination device such as a printer must therefore be moved to lie within the printer gamut. This will also apply if the dynamic ranges are mismatched between source and destination. Techniques for performing all of the processes are sophisticated and proprietary and reside in vendor specific CMMs.
Defining Terms
Addressability: The spacing of the dots on the page, specified in dots per unit length. This may be different in horizontal and vertical axes and does not imply a given dot diameter.
CIE 1931 standard observer: Set of curves obtained by averaging the results of color matching experiments performed in 1931 for noncolor defective observers. The relative luminances of the colors of the spectrum were matched by mixtures of three spectral stimuli. The curves are often called color matching curves.
Commision Internationale d’Eclairage (CIE): International standards body for lighting and color measurement. Central Bureau of the CIE, a-1033 Vienna, P.O. Box 169, Austria.
Digital halftone: Halftone technique based on patterns of same size dots designed to simulate a shade of gray between white paper and full colorant coverage.
Grayscale: Intrinsic modulation property of the marking technology that enables either dots of different size or intensity to be printed.
Halftone: Technique of simulating continuous tones by varying the amount of area covered by the colorant. Typically accomplished by varying the size of the printed dots in relation to the desired intensity.
H and D curve: Characteristic response curve for a photosensitive material that relates exposure to produced/developed optical density.
Resolution: Spacing of the printer dots such that full ink coverage is just obtained. Calculated from the dot size and represents the fundamental ability of the printer to render fine detail.
Saturation: When applied to color it describes the colorfulness with respect to the achromatic axis. A color is saturated to the degree that it has no achromatic component.
References
Cornsweet, T.N. 1970. Visual Perception. Academic Press, New York.
Diamond, A.S., ed. 1991. Handbook of Imaging Materials. Marcel Dekker, New York.
Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA.
Hunt, R.W.G. 1992. Measuring Color, 2nd ed. Ellis Horwood, England.
Hunt, R.W.G. 1995. The Reproduction of Color, 5th ed. Fountain Press. England.
Scharfe, M. 1984. Electrophotography Principles and Optimization. Research Studies Press Ltd., Letchworth,
Hertfordshire, England.
Schein, L.B. 1992. Electrophotography and Development Physics, 2nd ed. Springer–Verlag, Berlin.
Schreiber, W.F. 1991. Fundamentals of Electronic Imaging Systems, 2nd ed. Springer–Verlag, Berlin.
Ulichney, R. 1987. Digital Halftoning. MIT Press, Cambridge, MA.
Williams, E.M. 1984. The Physics and Technology of Xerographic Processes. Wiley-Interscience, New York.
Further Information
Color Business Report: published by Blackstone Research Associates, P.O. Box 345, Uxbridge, MA 01569- 0345. Covers industry issues relating to color, computers, and reprographics.
International Color Consortium: The founding members of this consortium include Adobe Systems Inc., Agfa-Gevaert N.V., Apple Computer, Inc., Eastman Kodak Company, FOGRA (Honorary), Microsoft Corporation, Hewlett-Packard Journal, 1985. 36(5); 1988. 39(4) (Entire issues devoted to Thermal Ink Jet). Journal of Electronic Imaging: co-published by IS&T and SPIE. Publishes papers on the acquisition, display, communication and storage of image data, hardcopy output, image visualization, and related image topics. Source of current research on color processing and digital halftoning for computer printers.
Journal of Imaging Science and Technology: official publication of IS&T, which publishes papers covering a broad range of imaging topics, from silver halide to computer printing technology.
The International Society for Optical Engineering, SPIE, P.O. Box 10, Bellingham, Washington 98227- 0010, sponsors conferences in conjunction with IS&T on electronic imaging and publishes topical pro- ceedings from the conference sessions.
The Hardcopy Observer, published monthly by Lyra Research Services, P.O. Box 9143, Newtonville, MA 02160. An industry watch magazine providing an overview of the printer industry with a focus on the home and office.
The Society for Imaging Science and Technology, IS&T, 7003 Kilworth Lane, Springfield, VA 22151. Phone (703)642 9090, Fax (703)642 9094. Sponsors wide range of technical conferences on imaging and printing technologies. Publishes conference proceedings, books, Journal of Electronic Imaging, Journal of Imaging Science and Technology, IS&T Reporter.
The Society for Information Display, 1526 Brookhollow Drive, Ste 82, Santa Ana, CA 92705-5421, Phone (714)545 1526, Fax (714)545 1547. Cosponsors annual conference on color imaging with IS&T.